The invention pertains to a thermo-optical switch comprising at least one input channel, one output channel, two switching channels, and one heating element.
Such a switch is known from N. Keil, et al., "(2.times.2) digital optical switch realised by low cost polymer waveguide technology," Electronics Letters, Vol. 32, No. 16 (Aug. 1, 1996), 1470-1471. This article describes a thermo-optical switch which has four waveguides (or, more accurately, waveguide channels): two input ports and two output ports, and four electrodes (or, more generally, elements) for heating the waveguides. Switching a signal from one of the input ports to one of the output ports is effected via selective heating of the waveguides.
By so-called "switching channels" are meant those channels over which a temperature difference is induced, which difference results in switching. In the 2.times.2 switch described above the input and the output channels also serve as switching channels. In a Mach-Zehnder interferometer (MZI), which in its simplest form (e.g., as a 1.times.1 or 2.times.2 switch) is made up of one 3 dB splitter and one 3 dB combiner connected by two channels, a temperature difference is induced over these channels which, e.g. in the case of a 1.times.1 switch, results in a switching action. Hence, in an MZI these channels serve as switching channels.
Thermo-optical digital (mode evolution) switches, such as the very common 1.times.2 "Y-branched" switch, make it possible to switch signals from the input port to one of the output ports by heating just one of the output channels. In this way a temperature difference, and a difference in refractive index, is induced between the output channels. A signal launched in the input channel in the zero-ordered mode will adiabatically evolve to the local fundamental mode of the waveguide having the highest effective refractive index, N.sub.eff. The effective refractive index difference between the two branches, .DELTA.N.sub.eff, is responsible for the switching action and is proportional to the difference in effective temperature, .DELTA.T.sub.eff (.DELTA.N.sub.eff =c.multidot..DELTA.T.sub.eff wherein "c" stands for the thermo-optical coefficient. "c" is negative for most polymers and positive for most inorganic materials). T.sub.eff is defined as the overlap between the normalised intensity profile of the local mode and the temperature profile induced by the heating element. The terms given above are known to the skilled person and do not require further elucidation.
In the case of polymeric adiabatic mode evolution switches the signal will propagate through the (comparatively) cool waveguide where the effective refractive index is highest.
Whether a switch is in the switched state may be dependent on the specifications of the switch or on the system of which the switch is part. For instance, in the case of a 1.times.2 switch, it may be necessary that after completion of the switching procedure less than 3% of the overall power of the outgoing signals passes through the output port which is qualified as being in the "off" state, while over 97% of the signal passes through the output port which is qualified as being in the "on" state. In the case of such a ratio the switch has to be able to attain an isolation of at least 15 dB (10 log 97/3).
It will be obvious that the term "switched" does not so much refer to an absolute physical state but rather indicates that the present switch meets the requirements (in this case especially the isolation) of the switched state. For instance, it may be that a switch will allow an isolation of 30 dB, while 18 dB suffices for a particular use. Generally speaking, 18 dB will be normative in that case.
FIG. 1 shows a top view of a 1.times.2 digital, planar thermo-optical mode evolution switch known in itself with one input channel (1), two output channels (2,3), and two resistive elements (4,5) for heating the output channels (2,3), which elements lie directly over the output channels, have the same width as the output channels, and are both provided with means to supply power (not shown here). The figure further shows that at the level where the two output channels (2,3) bifurcate (indicated by bisector B) the heating elements do not run directly above these channels but have been shifted in the lateral (transverse or X-) direction. This will result in a gradual, and hence adiabatic, setting in of the thermo-optical effect.
For clarity of the figure a substantially smaller scale has been chosen for the longitudinal or Z-direction than for the transverse or X-direction. In other words, there is question of a so-called "aspect ratio," which in this case is about 1:60 (X:Z) and which makes the switch appear much shorter than it actually is. The letter "S" indicates the separation between the centres of the output channels. The separation gradually increases in the longitudinal or Z-direction from the bifurcation.
To attain the switched state one of the resistive heating elements (4,5) is driven at a certain voltage, e.g., 5 Volt. This voltage is then kept constant in order to maintain the required temperature difference between the output channels, and thus the switched state. Hence there is a constant supply of heat to the switch. It is understood that even with regular switching (e.g., about once every 10 seconds) the total flow of heat to the switch will be more or less constant.
By reducing this power supply the lifespan of thermo-optical switches, which is restricted, int. al., by ageing of the optical material of which the switch is made, e.g., polymer, could be increased. Ageing is often attended with a change in the optical properties (in particular the refractive index) of the material, which may lead to the aforementioned isolation of the switch in the switched state being decreased, so that in time the switch will no longer be satisfactory and will have to be replaced. It also holds that a reduction of the supplied power will result in a lowering of the power density in the heating element (or heating elements), which benefits the lifespan and the reliability of both this element and the switch itself.
In addition, at a lower power the equipment driving the switch can have a less expensive design.